1. Field
The disclosed concept relates generally to electrical switching apparatus and, more particularly, to electrical switching apparatus, such as circuit breakers. The disclosed concept also relates to trip mechanisms for electrical switching apparatus.
2. Background Information
Electrical switching apparatus, such as circuit breakers, provide protection for electrical systems from electrical fault conditions such as, for example, current overloads, short circuits, abnormal voltage and other fault conditions. Typically, circuit breakers include an operating mechanism which opens electrical contact assemblies to interrupt the flow of current through the conductors of an electrical system in response to such fault conditions as detected, for example, by a trip mechanism, such as a trip unit.
Latches are an important part of electrical switching apparatus, such as circuit breakers. A latch assembly typically includes three components: a pivotable D-shaft, a latch plate and a latch shaft. The latch plate and the latch shaft are suitably joined together and the latch plate rotates about a longitudinal axis of the latch shaft. The D-shaft blocks or allows movement of the latch plate through a D-shaft slot when in a corresponding suitable axial position as the D-shaft rotates on its longitudinal axis. The latch shaft and the D-shaft both rotate about their respective longitudinal axes, which are disposed a fixed distance apart. The latch assembly can only rotate when the D-shaft is suitably oriented to allow the latch plate to pass through the D-shaft slot. The D-shaft can include a number of arms for interface, for example, by a trip plunger of a trip solenoid that is energized in response to a trip condition, as detected by the trip unit.
During an interruption event, the trip latch load is increased during excessive current flow because of magnetic forces generated on the operating mechanism and the movable contact arms. These magnetic forces translate to the operating mechanism and create the need for relatively higher trip latch forces. For example, sometimes the contact carrier can begin to open prior to the trip unit tripping the circuit breaker. In other words, the magnetic forces lift the contact carrier and require an additional latch force to trip open the operating mechanism. As a result, during a relatively high fault current, the trip solenoid must either have sufficient operating force to operate the trip latch lever, or the desired trip might be delayed or inhibited, which would be completely undesirable.
There is room for improvement in electrical switching apparatus.
There is also room for improvement in trip mechanisms for electrical switching apparatus.